1. Field of the Invention
The present invention relates to a magnetic particle and a production method thereof, as well as a magnetic recording medium containing the magnetic particles in a magnetic layer and a production method thereof.
2. Description of the Related Art
In order to increase magnetic recording density, the size of magnetic particles contained in a magnetic layer needs to be reduced. For example, when magnetic recording media widely used in the form of a videotape, a computer tape, a disk or the like contain ferromagnetic substance of equal weight, a reduction in particles size of the ferromagnetic substance will reduce the level of noise.
A material for magnetic particles which is effective in improving magnetic recording density can be a CuAu-type or Cu3Au-type hard magnetic ordered alloy (e.g. refer to Japanese Patent Application Laid-Open (JP-A) No. 2003-73705). The hard magnetic ordered alloy is known to have high crystal magnetic anisotropy due to strains occurring in an ordering process, and displays hard magnetism even when the size of magnetic particles is made small.
Magnetic particles displaying hard magnetism can be produced by a liquid-phase method and a vapor-phase method, and magnetic particles produced immediately after a liquid-phase method have a disordered phase and a face-centered cubic lattice structure.
This face-centered cubic lattice generally indicates soft magnetism or paramagnetism. Magnetic particles having soft magnetism or paramagnetism are not suitable for use in recording media. In order to obtain a hard magnetic ordered alloy with a coercive force of 95.5 kA/m (1,200 Oe), required for a magnetic recording medium, it is necessary to carry out annealing at a temperature not lower than a transformation temperature at which a disordered phase is transformed to an ordered phase.
When the above-mentioned magnetic particles are produced by a liquid-phase method, in order to prevent the metal of the magnetic particles from being oxidized, it is necessary to anneal the magnetic particles in a non-oxidizing atmosphere such as Ar, or N2. According to experiments performed by the inventors of the present invention, when an alloy phase is ordered by annealing, the transformation temperature may become high. The rise in transformation temperature is problematic from the viewpoints of heat resistance of a substrate, production facilities, and reproducibility of magnetic characteristics of the resultant magnetic recording medium.